The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to an improved apparatus for improving the efficiency of divertors towed by seismic vessels in water.
Cables are towed through water in marine seismic operations to transport acoustic energy sources, flotation buoys, hydrophones, and other marine seismic equipment through the water. For large seismic vessels, multiple cables are simultaneously towed in a wide swath through the water. Each cable may extend thousands of meters behind the seismic tow vessel, and adjacent cables must be separated to prevent cable entanglement during vessel movement and turning maneuvers.
Marine seismic operations typically survey the geologic formations underlying large geographic areas. Efficient vessel operation encourages large tow arrays with multiple cables and associated seismic equipment. Marine divertors attached to the cable arrays pull exterior cables outwardly from the in-line tow direction as the tow vessel moves through the water. Such divertors, also known as paravanes, maintain relative spacing between adjacent cables in a direction transverse to the in-line tow direction. Such spacing limits cable entanglement and establishes the transverse location between adjacent source and hydrophone arrays.
Conventional divertors are described in U.S. Pat. No. 3,611,975 to Ashbrook (1971) and in U.S. Pat. No. 4,033,278 to Waters (1977). Because conventional divertors require significant tow force during vessel movement, there is a need to improve tow efficiency by increasing fuel savings. Less drag also increases the seismic array tow capacity of each vessel and permits wider arrays to be towed during each vessel pass.
Floats and buoys support cables and other equipment in the water during marine seismic operations. U.S. Pat. No. 4,549,499 to Huffhines et al. (1985) disclosed a float integral with a V-shaped frame. U.S. Pat. No. 4,890,568 to Dolengowski (1990) disclosed a remotely controllable tail buoy. U.S. Pat. No. 4,676,183 to Conboy (1987) and U.S. Pat. No. 5,532,975 to Elholm (1996) disclosed floats for supporting paravanes in water.
Various divertor wings have been tested. Divertor vanes essentially comprise a wing in the water for urging cables outwardly from the in-line tow direction. For example, a divertor attached to a float was disclosed in U.S. Pat. No. 5,357,892 to Vatne et al. (1994), and techniques for anchoring paravanes was disclosed in U.S. Pat. No. 4,574,723 to Chiles et al. (1986). Because divertors function in a substantially vertical plane, divertors require buoyancy to maintain the relative position in a horizontal plane under tow and when the vessel is stopped. Buoyancy has traditionally been provided by surface xe2x80x9ctorpedo floatsxe2x80x9d or xe2x80x9crocket floatsxe2x80x9d attached with wire rope or chains to each divertor. Another float design was disclosed in U.S. Des. Pat. No. D297004 to Henriksen (1988). Such floats are aligned to the direction of water flow and do not add any lift to the suspended vane.
Certain improved divertors integrate floats within a body attached to divertor vanes. One example is illustrated in U.S. Pat. No. 4,484,534 to Thillaye du Boullay (1984), which disclosed hollow caissons having a rounded leading edge and a central rib. Such integrated floats stabilize movement of the divertor vanes and provide additional lift to the vanes under tow, and the total lift-to-drag ratio for conventional vanes and integrated floats is approximately 1.5 to 3.0. Other cable buoyancy systems were disclosed in U.S. Pat. No. 3,794,965 to Charske (1974) and in U.S. Pat. No. 4,252,074 to Blaisdell (1981).
A need exists for an improved divertor which reduces drag in the water while increasing the lift-to-drag ratio. Such divertor should adequately buoy the divertor vanes at rest and should maximize tow efficiency during marine seismic operations.
The invention provides an apparatus for providing floatation to a marine divertor in water. The apparatus comprises a hollow body attached to the divertor for providing positive buoyancy to the divertor when said divertor is moved through the water, and a curved leading end of the body substantially oriented at an angle facing the direction of the divertor movement through the water.
Another embodiment of the invention describes an apparatus for urging a tow cable transverse to the travel direction of a tow vessel in water. Such embodiment comprises a body attached to the tow cable, a divertor vane attached to the body for urging the body and attached tow cable at an angle transverse to the tow vessel travel direction, and a float having a curved leading end and being attached to the body for providing positive buoyancy to the body when the body is moved through the water.